1. Field of the Invention
This invention relates to a synthetic aperture radar system mounted on a moving radar platform, such as an aircraft or the like, for providing an image of a stationary object on the surface of the earth or sea.
2. Prior Art
FIG. 1 diagrammatically shows a conventional synthetic aperture radar system.
The conventional radar system, as shown in FIG. 1, comprises a transmitter 1, a transmitting antenna 2 for emitting radio waves to a stationary object constituting a target to be observed, a receiving antenna 3 for receiving echo signals reflected back from the target, a receiver 4, a pulse compression unit 5 for enhancing the resolution in the lengthwise direction of the received signals (range resolution), an azimuth compression section 6 for achieving high resolution in the azimuth direction (cross-range resolution), a display 7 for visually presenting images obtained from the radar system and a reference signal generator 8 for providing the azimuth compression section 6 with data showing the history of changes in Doppler frequency with time as a reference signal for the azimuth compression. The azimuth compression section 6 includes a first Fourier transform unit 61 for carrying out the Fourier transform of the output of the pulse compression unit 5, a second Fourier transmission unit 62 for carrying out the Fourier transform of the output of the reference signal generator 8, a complex multiplication unit 63 for carrying out the complex multiplication of the outputs of the first and second Fourier transform units 61 and 62, and an inverse Fourier transform unit 64 for carrying out the inverse Fourier transform of the output of the complex multiplication unit 63. The transmitter 1 and the transmitting antenna 2 constitute a transmission means 9 and the receiving antenna 3 and the receiver 4 constitute a reception means 10.
By such an arrangement, the radio wave emitted from the transmitter 1 via the transmitting antenna 2 towards the target to be observed is reflected back by the target as an echo signal to the receiving antenna 3.
The received echo signal is directed through the receiver 4 to the pulse compression unit 5 in which a pulse compression operation is achieved for enhancing the range resolution.
As an example for achieving such pulse compression, there is known a system including a matched filter having frequency-delay time characteristics, wherein a radio wave linearly frequency-modulated within the pulse width of the transmission pulse is transmitted and the returned echo signal is passed through the matched filter.
The echo signal subjected to the pulse compression is directed to the azimuth compression section 6 in which the azimuth compression is carried out by the first and second Fourier transform units 61, 62, the complex multiplication unit 63 and the inverse Fourier transform unit 64, whereby the cross-range resolution is improved on the basis of the reference signal output by the reference signal generator 8.
In the case of an aircraft travelling with a speed v and emitting radio waves in a downward direction perpendicular to the direction of motion, the Doppler effect caused by the relative movements between the aircraft and the ground is utilized so that the cross-range resolution in the direction of the flight path can be enhanced by obtaining a correlation between a series of the received signals and the reference signal designating the history of variation with time of the Doppler frequency.
In this manner, the image information obtained with range and cross-range resolutions enhanced is displayed on the display 7.
With such a conventional synthetic aperture radar system as described above, the cross-range resolution .DELTA..gamma. can be obtained by the following expression using the antenna beam width .theta..sub.B and transmission wave length .lambda.. ##EQU1## The bandwidth B of the Doppler frequency of the received signal can also be obtained by: ##EQU2## In order to enhance the resolution .DELTA..gamma., therefore, it is necessary to increase the beam width .theta..sub.B. However, as the beam width .theta..sub.B increases, the necessary frequency bandwidth B is increased to require a higher pulse repetition frequency PRF (PRF&gt;B), and the maximum observation distance R.sub.max without range ambiguity is limited according to the following expression using the velocity of light C and the moving velocity of the radar v. ##EQU3##
Thus, the conventional system was subjected to problems such that if it was intended to enhance the resolution, a far distant region could not be observed and if it was intended to make far distant observation, the resolution was reduced.